Coil

Whats the best way to see if a coil is operating properly?
It's in a 1974 CJ-5 with a Pertronix ignition, which I left the key on and fried, got a new one and still won't fire. My thinking is it might have also fried the coil but I'd like to check it out first before spending the bucks on a new one. It's a Flamethrower 40,000v 1.5 ohm coil.

Jeff

 

Jeff, I did a check on mine today, but it's a 79 so it may not apply to yours.

It's always best to check a coil warm. First check for oil leaks and carbon tracks. Then disconnect the coil connector and check for resistance between the + - terminals, should be 1.13 to 1.23 ohms@ 75F. If your engine had been running 1.5 ohms @ 200F. If your not in that range replace the coil, if you are with in range check the ohms from the center tower and either + - and if your still in the range the coil is good.

 

I thought I had a bad coil a few weeks ago. My engine quit while driving down the road. Couldnt restart it, found out I was'nt getting a spark from the coil. Since it's 32 years old, I figured it was bad. Called a buddy, He bought one a AZone and came out to my rescue. Installed the new coil and the moter fired right up. I then turn the moter off to tighten up the coil clamps, closed the hood, turned the key and it would'nt start. Openned up the hood and while I turned the moter over, my buddy wiggled the coil to distributor wire and the engine started. The wire was bad.
That wire gets so hot, and after 32 years it was broken down. We replced the wire and all is well. Now I have a spare coil.

 

Just a comment - first, checking the LT (low tension) coil resistance should work, but most DVMs that people own aren't nearly that accurate at low resistances. Measuring 1.18 +/- 0.05 ohms is really optimistic unless you have a meter specifically designed to measure low resistances. You can easily add or subtract 0.1 ohm depending on the condition of your probes, whether you touch the probes or not with your fingers (extra current path through your body), and so forth.

I think that all you can hope for is testing continuity, both from the +/- terminals and from the center terminal to the case, and shorts between the terminals and the case. The outside of the coil (the can) must be well grounded, since the current for the spark must return to the coil through the engine block and the coil can.

If you suspect the coil, I'd suggest you swap in a known good coil as a test. Even if you have to buy a coil, and there's some other problem, having a spare is worthwhile. Coils can fail all at once, and it's a spare you should carry.